How is MuZero's second binary plane for chess defined?
Is the second binary plane all zeros or all ones? Or, something else? How is it known if the move is off the board? For my game, I know if it is a legal move on the board, but do not know if the move is off the board.

The second binary plane is one-hot by definition, there is a single one and everything else is zero. If this definition is not met, it's no longer &quot;one-hot&quot;.
The paper doesn't state how exactly to implement &quot;off the board&quot; implementation. Research paper wouldn't go into coding level. However, detecting &quot;off the board&quot; is not a challenging task.

https://webcache.googleusercontent.com/search?q=cache:djj-G4T_PwgJ:https://craftychess.com/hyatt/boardrep.html+&amp;cd=2&amp;hl=en&amp;ct=clnk&amp;gl=au


The next step in board representation evolution is to enclose the board inside a larger array, so that illegal squares are &quot;off&quot; the edge and are easily detectable.

A possibility is add borders to your board. Crafty did that. Extend the board to 10x10. Not 9x9 because you need to deal with knight jumping.
Exactly how you should be doing it is implementation defined. We don't know what Google did because AlphaZero is not open source. I'm here just giving you an example.